1. Field of the Invention
This invention relates to a method of reducing iron loss of a grain oriented silicon steel sheet used in transformers and the like.
2. Related Art Statement
The iron loss of grain oriented silicon steel sheet is the heat energy loss generated in the sheet when using it as a core of a transformer or the like. Lately, the demand for reducing the heat energy loss or iron loss of the grain oriented silicon steel sheet has become higher in view of energy circumstances.
In order to reduce the iron loss, there have been attempted various methods, for example, a method wherein crystal grains of the steel sheet are highly oriented in {110}&lt;001&gt; orientation, a method wherein the Si amount is increased to raise the electrical resistance of the steel sheet, a method of reducing the impurity amount, a method of thinning the thickness of the steel sheet, and the like. However, the reduction of iron loss by these metallurgical methods has substantially reached its limit.
Therefore, there have been proposed various methods for the reduction of iron loss other than the above metallurgical methods. Among them, a method of reducing iron loss by irradiation with a pulse laser as described in Japanese Patent Application Publication No. 57-2,252 or the like, is industriallized at the present. Although this method makes it possible to largely reduce iron loss as compared with the case of using a conventional metallurgical method, it is difficult to avoid the increase of initial cost and running cost due to the fact that the apparatus used is expensive and the life of the lamp used for excitation of the laser is not so long. Further, the laser beam used is not often a visible light, so that it is always necessary to take a countermeasure from the viewpoint of safety.
Furthermore, in the above laser irradiation method, a strain which causes refinement of the magnetic domain is introduced by shock wave reaction due to the evaporation of surface coatings and a part of the base metal by the irradiation, so that it is required to repair the surface coatings by recoating. If the recoating is performed, the lamination factor inevitably becomes poor and the magnetic properties in the actual application are degraded. Moreover, if the base metal is excessively evaporated, the magnetic flux density of the steel sheet undesirably lowers.
In Japanese Patent laid open No. 59-33,802 and No. 59-92,506 is disclosed a method of irradiating a continuous laser beam, but this method has drawbacks that the effect of reducing iron loss is small, and the absorption rate of the laser beam by the steel sheet inevitably changes to make the effect variable in addition to the drawbacks similar to those described on the pulse laser method.
As a method substituting for the above methods, the inventors have previously proposed a method of irradiating a plasma flame to the surface of the steel sheet and filed as Japanese Patent Application No. 60-236,271. According to this method, the repairing of the surface coatings as in the pulse laser method is not required and also the base metal is not evaporated, so that a high lamination factor can be maintained. On the other hand, in case of laser beam irradiation, the absorption of laser beam becomes a problem, resulting from the inevitable change of color in the surface coating on the steel sheet or an inevitable change of absorption coefficient, and consequently the laser irradiation effect is not constant. On the contrary, in case of plasma flame irradiation, the plasma flame is directly irradiated to the steel sheet, so that a stable effect is obtained even if the color of the steel surface is fluctuated, and consequently the iron loss values after the irradiation is low as compared with that experienced after laser irradiation.